System and method for  purification of silane using liquid nitrogen in a polysilicon production process

ABSTRACT

A system and method for improved cryogenic cooling of process streams in polysilicon manufacturing is provided. The disclosed system and method provides for the cryogenic cooling of a silane and hydrogen process stream during the manufacture of polysilicon with concurrent recovery of refrigeration capacity from the vaporized nitrogen as well as the recovery of refrigeration capacity from the cold hydrogen stream. The improved cryogenic cooling system and method reduces the overall consumption of liquid nitrogen without sacrificing cooling performance of the cryogenic cooling of the silane and hydrogen process stream.

CLAIM OF PRIORITY

The present invention is a National Stage Entry of PCT/US2011/059711 filed Nov. 8, 2011, which claims priority from Provisional Application No. 61/414,702 filed Nov. 17, 2010. No new matter has been added.

FIELD OF THE INVENTION

The present system and method relates to cryogenic cooling of intermediate process streams during the polysilicon production processes, and more particularly to methods and systems for the refrigeration recovery during purification of silane in a fluidized bed polysilicon production process.

BACKGROUND

There have been developed numerous processes for the production of polysilicon including the classic Siemens process, the Hemlock process, the Ethyl Corp. developed fluidized bed process, the Union Carbide process, and the Komatsu process. In the commonly used Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150° C. The trichlorosilane gas decomposes and deposits additional silicon onto the electrically heated silicon rods, enlarging them according to chemical reactions such as:

2HSiCl₃→Si+2HCl+SiCl₄

Silicon produced from this and similar processes are called polycrystalline silicon. Because of the high resistivity of the silicon seed rods, the Siemens process requires two power supplies—one for preheating the rods into a conductive state, and the second for superheating the rods by conduction. Most of the energy from the hot silicon rods is radiated into water-cooled bell jars covering the Siemens reactor.

In the fluidized bed process for the manufacture of polysilicon, pure silicon pellets are grown from tiny pure silicon seeds into polysilicon granules in a high-temperature reaction vessel. This process uses silicon fluoride as a precursor material to produce silane, SiH₄. Silicon fluoride is readily available and relatively inexpensive waste byproduct of various industries. An overview of a fluidized bed process for production of polysilicon is depicted in FIG. 1.

The fluidized bed process of manufacturing polysilicon offers some significant economic advantages compared to the Siemens process for the production of polysilicon. The energy losses and hence the energy consumption are considerably reduced in the fluidized bed process because the decomposition operates at a lower temperature, and cooling the bell jar is not required. Another advantage in the fluidized bed process is that very large reactors may be constructed and operated continuously, reducing further the capital and operating costs. Unlike the Siemens process, the end products in the fluidized bed process for manufacture of polysilicon are small granules of polysilicon that may have some commercial advantages, such as when continuous feeding polysilicon into the customer's process is required.

In the fluidized bed process for production of polysilicon shown in FIG. 1, silicon fluoride is distilled into a gaseous feed of silane in hydrogen. After distillation of the silicon fluoride, the influent gaseous silane feed is purified/separated in a purification unit and thermally decomposed in a fluidized bed to produce polysilicon. Silicon seed particles are introduced into the fluidized bed sustained by a stream of silane and hydrogen. The silicon from the decomposed silane attaches to the seed particles in the fluidized bed reactor, which grow to granule sized pellets during their free fall to the bottom of the reactor. Prior to the introduction into the fluidized bed reactor, the influent gaseous feed comprising silane in hydrogen is purified/separated through a series of heat exchangers and economizers that use liquid and gaseous nitrogen to separate these intermediate process streams, namely into a hydrogen stream and a silane stream.

An example of the prior art means to separate the intermediate process streams in the fluidized bed polysilicon production process is depicted in FIG. 2. As seen therein, the incoming gaseous feed (12) of silane and hydrogen at a flow rate of about 975 kg per hour generally comprises about 2% silane (SiH₄) and 98% hydrogen (H₂) at a temperature of about 25° C. and a pressure of about 0.66 MPa. This incoming gaseous feed or process stream (12) is cooled in a series of heat exchangers and economizers to a prescribed final temperature where the silane and hydrogen are separated is phase separator.

This multi-step sequence of cooling the influent process stream (12) includes an economizer (13) which pre-chills the silane and hydrogen gas stream (12) to a temperature of about −80° C. using cold hydrogen gas (22). This pre-chilled silane and hydrogen stream (14) is then directed to a second, relatively small economizer (15) that further cools the pre-chilled silane and hydrogen stream (14) to an intermediate temperature of about −144° C. using gaseous nitrogen (32) at about −164° C. The resulting cooled silane and hydrogen stream (16) is directed to a cryogenic, heat exchanger (17) where it is cooled with liquid nitrogen at about −179° C. to the final prescribed process temperature of about −165° C. The fully cooled silane and hydrogen stream (18) is directed to a phase separator (19) where the silane is condensed to a liquid product (20) to be directed to the fluidized bed and the resulting cold hydrogen gas (22) at about −160° C. is directed back to the economizer (13) to pre-chill the influent process stream (12). The used hydrogen stream (24) is vented or used elsewhere in the plant.

The cooling medium for cryogenic heat exchanger (17) and economizer (15) is liquid and/or gaseous nitrogen flowing through the cooling circuit. The nitrogen used in the polysilicon purification process originates from a source of liquid nitrogen (not shown). The liquid nitrogen is at about −179° C. and a pressure of 0.5 MPa is supplied to the cryogenic heat exchanger (17) at a flow rate of between about 1150 to 1500 kg per hour where it cools the silane and hydrogen process stream (16) to the final prescribed temperature of about −160° C. The nitrogen stream (32) at about −164° C. exiting the cryogenic heat exchanger (17) is routed to the economizer (15) where it pre-chills the silane and hydrogen process stream (14) and exits the economizer (15) in a gas stream (34) at about −130° C. The 1500 kg per hour nitrogen gas stream (34) is eventually directed to another gas to air heat exchanger (35) where the cold nitrogen gas (34) is warmed to an exhaust or vent temperature of about 10° C. at a pressure of about 0.3 MPa using incoming warm air (38) which exits the heat exchanger (35) as cold air (40).

The cost associated with the separation and purification of the silane using the above-described process is significant in view of the large amounts of high pressure nitrogen used as the cooling medium. Using the above described prior art purification and separation process, about 1150 kg per hour of high pressure liquid nitrogen is consumed and much of the refrigeration capacity of the nitrogen is not recovered or not used within the cryogenic cooling system.

What is needed therefore is improved means for separation and purification of a gaseous silane/hydrogen feed in the fluidized bed process for production of polysilicon that utilizes less liquid nitrogen or lower pressure liquid nitrogen, or both.

SUMMARY OF THE INVENTION

The present invention may be characterized as a method for cryogenic cooling of a silane in hydrogen process stream in the production of polysilicon, the method comprising the steps of (a) pre-chilling a process stream of silane in hydrogen using using a cooling stream and one or more economizers; (b) cooling the pre-chilled process stream with liquid nitrogen in a cryogenic heat exchanger to a prescribed final temperature; (c) separating the cooled process stream at the prescribed final temperature into a product of liquid silane and a cold hydrogen stream; (d) recycling the cold hydrogen stream to form part of the cooling stream in the one or more economizers to pre-chill the process stream; (e) forcibly directing a portion of the used hydrogen stream from one or more of the economizers to an auxiliary heat exchanger; (f) directing the nitrogen stream from the cryogenic heat exchanger to the auxiliary heat exchanger to re-cool the used hydrogen stream; and (g) directing the re-cooled, used hydrogen stream to form part of the cooling stream in the one or more economizers to pre-chill the process stream. The excess refrigeration capacity of the cold hydrogen stream is directly transferred to the process stream flowing through at least one of the one or more economizers and the excess refrigeration capacity of the nitrogen stream is indirectly transferred to the process stream flowing through at least one of the one or more economizers.

The present invention may also be characterized as a cryogenic cooling system comprising: (i) a process stream of silane in hydrogen; (ii) a source of liquid nitrogen; (iii) a cryogenic heat exchanger for cooling the process stream using the liquid nitrogen; (iv) a phase separator disposed downstream of the cryogenic heat exchanger, the phase separator adapted for separating the cooled process stream into a product of liquid silane and a cold hydrogen stream; (v) one or more economizers for pre-chilling the process stream with the cold hydrogen stream, the one or more economizers disposed upstream of the cryogenic heat exchanger; (vi) a first recycle conduit coupling the outlet of the phase separator to the one or more economizers to direct the cold hydrogen stream from the phase separator to at least one economizer to pre-chill the process stream; (vii) a second heat exchanger coupled to the cryogenic heat exchanger and adapted for using nitrogen exiting from the cryogenic heat exchanger to cool a used hydrogen stream; (viii) a second recycle conduit coupling the outlet of at least one economizer through the second heat exchanger and to either the first recycle conduit or the inlet of at least one economizers to pre-chill the process stream; (ix) a blower disposed in operative association with the second recycle conduit to forcibly drive the used hydrogen stream from the outlet of at least one economizers through the second heat exchanger and to either the first recycle conduit or the inlet of at least one economizer. In the present cryogenic cooling system, the excess refrigeration capacity of the nitrogen stream exiting from the cryogenic heat exchanger is transferred first to the used hydrogen stream flowing through the second heat exchanger and subsequently to the process stream flowing through one or more economizers and the excess refrigeration capacity of the cold hydrogen stream exiting the phase separator is directly transferred to the process stream flowing through one or more of the economizers.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present invention will be more apparent from the following, more detailed description thereof, presented in conjunction with the following drawings, wherein:

FIG. 1 is a schematic illustration of the fluidized bed process for production of poly silicon;

FIG. 2 is a schematic illustration of a prior art cryogenic cooling and separation system used in the fluidized bed process for production of polysilicon;

FIG. 3 is a schematic illustration of a preferred embodiment of the cryogenic cooling and separation system in accordance with the present invention;

FIG. 4 is a schematic illustration of an alternate embodiment of the cryogenic cooling and separation system;

FIG. 5 is an illustration of a three stream heat exchanger that can be used to achieve the refrigeration recovery associated with the present invention; and

FIG. 6 is an illustration of an alternate concept for a heat exchanger that can be used to achieve the refrigeration recovery associated with the present invention.

Turning now to FIG. 3, a preferred embodiment of the present cryogenic cooling system (50) and method is shown. As seen therein, the incoming gaseous feed (52) of silane and hydrogen at a prescribed flow rate of about 975 kg per hour generally comprises about 2% silane (SiH₄) and 98% hydrogen (H₂) at a temperature of about 25° C. and a pressure of about 0.66 MPa. Much like the prior art purification process, this incoming gaseous feed or process stream (52) is cooled in a series of economizers and heat exchangers to a prescribed final temperature where the silane and hydrogen are separated in a phase separator (59).

This preferred process of cooling the influent process stream includes first pre-chilling the silane and hydrogen influent stream (52) in an economizer (53) to a temperature of about −80° C. using a cooling stream including the cold hydrogen stream. This pre-chilled silane and hydrogen stream (54) is then directed to a second economizer (55) that further cools the pre-chilled silane and hydrogen stream (54) to an intermediate temperature of −167° C. also using a cooling stream (64) that includes the cold hydrogen stream (62).

The resulting cooled silane and hydrogen stream (56) is directed to a cryogenic heat exchanger (57) where it is further cooled with liquid nitrogen at about −179° C. to a colder final prescribed process temperature of about −173° C. This fully cooled silane and hydrogen stream (58) is then directed to a phase separator (59) where the silane is condensed to a liquid product (60) to be directed to the fluidized bed and the resulting cold hydrogen stream (62) at about −122° C. is used in pre-chilling and cooling the influent and intermediate silane and hydrogen process streams (52,54) in the economizers (53,55) as described in more detail below.

The cryogen used in the cryogenic heat exchanger (57) is preferably liquid nitrogen (80) from a source of liquid nitrogen. The liquid nitrogen (80) is supplied to the cryogenic heat exchanger (57) at a flow rate of only 554 kg per hour, a temperature of about −179° C., and a pressure of 0.4 MPa where it cools the silane and hydrogen process stream (56) to a colder final prescribed temperature of about −173° C. The nitrogen stream (82) exiting the cryogenic heat exchanger (57) at about −164° C. is routed to an auxiliary heat exchanger (75) where it is used to provide re-cooling of the used hydrogen gas (76). Nitrogen gas (84) exiting the auxiliary heat exchanger (75) is then directed to another gas to air heat exchanger (85) where the nitrogen gas (84) at about 14° C. is warmed to an exhaust or vent temperature of about 25° C. at a pressure of about 0.3 MPa using incoming warm air (88) which exits the heat exchanger (85) as cool air (90).

The above-described used hydrogen gas (76) represents a portion of the warm hydrogen gas (72) exiting the economizer (53). The warm, used hydrogen gas (72) exiting the economizer (53) is preferably divided into two streams. One portion of the warm, used hydrogen gas (74) is vented or directed elsewhere in the plant whereas the second portion of the warm, used hydrogen gas (76) is recycled to the second or auxiliary heat exchanger (75) using a blower (73). This second portion of the warm, used hydrogen gas (76) is re-cooled in the auxiliary heat exchanger (75) using the nitrogen stream (82) exiting the cryogenic heat exchanger (57). The re-cooled, used hydrogen stream (78) is then combined with the cold hydrogen stream (62) from the phase separator (59). The combined hydrogen cooling stream (64) is directed to first to the economizer (55) to cool the intermediate silane and hydrogen stream (54) and then to the economizer (53) to pre-chill the warm, influent silane and hydrogen stream (52).

The excess refrigeration capacity of the nitrogen stream (82) exiting from the cryogenic heat exchanger (57) is transferred indirectly to the influent process stream (52) by first transferring the refrigeration capacity to the recycled, used hydrogen gas (76) flowing through the auxiliary heat exchanger (75), which, in turn, is subsequently used to pre-chill the influent and intermediate process streams (52,54) flowing through the economizers (53,55). In addition, the excess refrigeration capacity of the cold hydrogen stream (80) exiting the phase separator (59) is transferred directly to the influent and intermediate process streams (52,54) flowing through the economizers (53,55).

By using both direct refrigeration recovery and indirect refrigeration recovery, the influent process stream is cooled to a lower temperature. This, in turn, reduces the amount of nitrogen needed in the cryogenic heat exchanger to obtain the desired or prescribed final temperature for separation. The reduction in nitrogen consumption lowers the operating costs associated with the present cryogenic cooling system and process when compared to the prior art cooling arrangements.

For example, when comparing the presently disclosed cryogenic cooling system and method for purification of silane in the fluidized bed polysilicon production process disclosed herein against the prior art cryogenic cooling system and method, it is apparent that significant operating cost savings in terms of reduced cryogen consumption and lower operating pressures can be achieved. The reduction in cryogen consumption alone should allow the plant to realize between about 20% and 50% improvement without sacrificing or curtailing the purification of silane or the production of polysilicon.

Turning now to FIG. 4, there is shown a schematic illustration of an alternate, more generic embodiment of the present cryogenic cooling and separation system. In this more generic version of the cryogenic cooling system, the influent or feed process stream (152) is a gaseous stream of silane in hydrogen which is cooled in a multi-step process and fully cooled process stream (158) at about −173° C. is subsequently separated into liquid silane stream (160) and a hydrogen stream (162). The cooling and separation of the influent or feed process stream (152) is accomplished using a first economizer (153) followed by a cryogenic heat exchanger (157) and then the phase separator (159).

The cryogen source used in the cryogenic heat exchanger (157) is preferably liquid nitrogen (180) delivered at approximately −179° C. and 0.4 MPa to cool the pre-chilled process stream prior to its separation. The nitrogen stream (182) exiting the cryogenic heat exchanger (157) at a temperature of about 464° C. is then directed to a second or auxiliary heat exchanger (175) where it cools the warm hydrogen stream (176) (i.e., hydrogen gas). The spent gaseous nitrogen (186) at about 14° C. is subsequently vented to the atmosphere or released for other uses within the plant. This multi-step use of the cryogenic nitrogen recovers and utilizes a significant portion of the available refrigeration capacity of the cryogen.

The separation of the fully cooled process stream (158) of silane and hydrogen in the separator (159) produces a liquid silane stream (160) at −173° C. and cold hydrogen stream (162) at about −172° C. The cold hydrogen stream (162) is then recirculated to the economizer (153) for cooling the 25° C. influent or feed process stream (152) to an intermediate pre-chilled process stream (154). The used hydrogen gas (172) exiting the economizer (153) at a temperature of about 11° C. is divided into two streams. One portion of the used hydrogen stream (174) is vented or used elsewhere in the plant whereas the second portion of the used hydrogen stream (176) is forcibly recycled to the second or auxiliary heat exchanger (175) using a blower (173). This portion of the used hydrogen stream (176) is re-cooled in the second or auxiliary heat exchanger (175) to a temperature of about −147° C. using the cold nitrogen stream (182) exiting to cryogenic heat exchanger (157). The re-cooled, used hydrogen stream (178) is then combined with the cold hydrogen stream (162) from the phase separator (159). The combined hydrogen stream (164) is directed to the economizer (153) to directly cool the influent or feed process stream (152).

As with the previously disclosed preferred embodiments, the embodiment schematically illustrated in FIG. 4 provides improved refrigeration capacity recovery of both the cold hydrogen stream and the cryogenic stream. Specifically, the excess refrigeration capacity of the nitrogen stream exiting from the cryogenic heat exchanger is transferred indirectly to the influent process stream whereas the excess refrigeration capacity of the cold hydrogen stream exiting the phase separator is transferred directly to the influent process stream. This improved refrigeration recovery applied to the influent process stream of silane and hydrogen reduces the amount of nitrogen needed in the cryogenic heat exchanger to obtain the desired or prescribed final temperature for separation.

FIG. 5 is an illustration of a three stream integrated heat exchanger (200) that can be used to achieve the refrigeration recovery associated with the present invention. As seen therein, one of intakes to the heat exchanger (200) is the influent process stream of silane in hydrogen (252) with the corresponding outlet stream being the fully cooled process stream (258) which is directed to the phase separator (259). As discussed above, the phase separator (259) produces a liquid silane stream (260) at −173° C. and cold hydrogen stream (262) at about −172° C. The second intake stream to the three stream heat exchanger (200) is liquid nitrogen (280) at a temperature of about −179° C. and the corresponding outlet is the nitrogen gas (284) at a temperature of about 14° C. The third stream is the cold hydrogen gas (262, 270) and the corresponding outlet is the used hydrogen gas (272) at about 11° C. which can be vented to the atmosphere (274) or otherwise used within the plant.

FIG. 6 is an illustration of an alternate concept for an integrated heat exchanger (200) that, similar to the heat exchanger of FIG. 5, can also be configured to achieve the refrigeration recovery associated with the present invention. This embodiment is similar to that of FIG. 5, but further illustrates the recycling of the used hydrogen gas (276) via the blower (273) through the heat exchanger (200) to be combined with the cold hydrogen gas (262) to form a combined hydrogen stream (264).

Using the integrated heat exchangers schematically depicted in FIGS. 5 and 6, one can combine the heat exchange or transfer functions of the cryogenic heat exchanger and the second or auxiliary heat exchanger into a single integrated device. Similarly, one can also combine the heat exchange or transfer functions of one or both heat exchangers and the economizers into a single integrated device.

From the foregoing, it should be appreciated that the present invention thus provides an improved method and system for cryogenic cooling of a process stream. While the invention herein disclosed has been described by means of specific embodiments and processes associated therewith, numerous modifications and variations can be made thereto by those skilled in the art without departing from the scope of the invention as set forth herein or sacrificing all its material advantages. 

What is claimed is:
 1. A method for cryogenic cooling of a silane in hydrogen process stream in the production of polysilicon, the method comprising the steps of: pre-chilling a process stream of silane in hydrogen using a cooling stream and one or more economizers; cooling the pre-chilled process stream with liquid nitrogen in a cryogenic heat exchanger to a prescribed final temperature; separating the cooled process stream at the prescribed final temperature into a product of liquid silane and a cold hydrogen stream; recycling the cold hydrogen stream to form part of the cooling stream in the one or more economizers to pre-chill the process stream; forcibly directing some or all of the used hydrogen stream from the one or more economizers to an auxiliary heat exchanger; and directing the nitrogen stream from the cryogenic heat exchanger to the auxiliary heat exchanger to re-cool the used hydrogen stream; and directing the re-cooled, used hydrogen stream to form part of the cooling stream in the one or more economizers to pre-chill the process stream.
 2. The method of claim 1 wherein the cooling stream is comprised of a mixture of the cold hydrogen stream and the used hydrogen stream and wherein the excess refrigeration capacity of the cold hydrogen stream is directly transferred to the process stream and the excess refrigeration capacity of the nitrogen stream is indirectly transferred to the process stream via the used hydrogen stream.
 3. The method of claim 2 further comprising the step of adjusting the characteristics of the cooling stream by venting a portion of the used hydrogen stream prior to mixing with the cold hydrogen stream to alter the mixture of the cold hydrogen stream and the used hydrogen stream forming the cooling stream.
 4. The method of claim 1 wherein the cryogenic cooling of the silane in hydrogen process stream is controlled by adjusting the flow of the incoming silane in hydrogen process stream; the flow of liquid nitrogen through the cryogenic heat exchanger; and the flow of the used hydrogen gas through the auxiliary heat exchanger.
 5. A cryogenic cooling system comprising: a process stream of silane in hydrogen; a source of liquid nitrogen; a cryogenic heat exchanger for cooling the process stream using the liquid nitrogen; a phase separator disposed downstream of the cryogenic heat exchanger, the phase separator adapted for separating the cooled process stream into a product of liquid silane and a cold hydrogen stream; one or more economizers for pre-chilling the process stream h the cold hydrogen stream, the one or more economizers disposed upstream of the cryogenic heat exchanger; a first recycle conduit coupling the outlet of the phase separator to the one or more economizers to direct the cold hydrogen stream from the phase separator to the economizer to pre-chill the process stream; a second heat exchanger coupled to the cryogenic heat exchanger and adapted for using the nitrogen stream exiting from the cryogenic heat exchanger to cool a used hydrogen stream; a second recycle conduit coupling the outlet of the one or more economizers through the second heat exchanger and to either the first recycle conduit or the inlet of the one or more economizers to pre-chill the process stream; a blower disposed in operative association with the second recycle conduit to forcibly drive the used hydrogen stream from the outlet of the one or more economizers through the second heat exchanger and to either the first recycle conduit or the inlet of the one or more economizers; wherein excess refrigeration capacity of the nitrogen exiting from the cryogenic heat exchanger is transferred first to the used hydrogen stream flowing through the second heat exchanger and subsequently to the process stream flowing through the one or more economizers; and wherein excess refrigeration capacity of the Bold hydrogen stream exiting the phase separator is directly transferred to the process stream flowing through the one or more economizers.
 6. The system of claim 5 wherein the cryogenic heat exchanger and the second heat exchanger are integrated into a three stream heat exchanger wherein the nitrogen stream cools the pre-chilled process stream and the used hydrogen stream.
 7. The system of claim 5 wherein the cryogenic heat exchanger and the one or more economizers are integrated into a multi-stream heat exchanger wherein the cold hydrogen stream pre-chills the process stream and the nitrogen stream cools the pre-chilled process stream.
 8. The system of claim 5 wherein the cryogenic heat exchanger, the auxilliary heat exchanger and the one or more economizers are integrated into a multi-stream heat exchanger wherein the cold hydrogen stream pre-chills the process stream and the nitrogen stream cools the pre-chilled process stream and the used hydrogen stream.
 9. The system of claim 5 further comprising a third heat exchanger for transferring any remaining refrigeration capacity from the nitrogen stream to cool an air stream. 